This is the general term for any number of complex organic bases that are found in nature in seed-bearing plants. These substances are usually colorless but bitter to the taste. Alkaloids often contain nitrogen and oxygen and possess important physiological properties.

Examples of alkaloids include not only quinine, atropine, and strychnine but also CAFFEINE, NIC-OTINE, MORPHINE, CODEINE, and COCAINE. Therefore, many drugs that are used by humans for both medical and nonmedical purposes are produced in nature in the form of alkaloids. Naturally occurring receptors for many alkaloids have also been identi-fied in humans and otheranimals, suggesting an evolutionary role for the alkaloids in physiological processes.